This genus is classified in the tribe Heliantheae. It comprises 35–40 species, mostly found in the Americas. Those species previously considered to belong to the genus Franseria Cav. are loosely known as the franserioid ragweeds or false ragweeds. The "original" Ambrosia species are described as ambrosioid ragweeds or, simply, ragweeds. However, there remain several taxonomic problems that require clarification, and much confusion exists in the use of both scientific and colloquial names.

Ambrosia species are American in origin and are known in ecological terms as pioneer plants; that is to say they are well adapted to invade vacant soils from which the customary vegetation has been removed. From a limited distribution, they have thrived with the coming of agriculture and the interference with the ecology of the North American prairies (Wodehouse 1971). The plants are still spreading and have invaded the Pacific North West (Perlman 1952). Ragweed has also been spreading in Europe since the mid-20th century and is now a pest in several countries of Eastern Europe and rapidly expanding in Central Europe. It is estimated that 4–5% of Europeans have become sensitised to the highly allergenic pollen (Richter et al. 2013).

In Hawaii, short ragweed (Ambrosia artemisiifolia L.) is reported to be present in a number of locations. The presence of A. psilostachya DC. in Australia was first reported by Ford (1963). Ragweed was first introduced from North America into Assam where it has now established itself as an escape (Behl et al. 1966). The plants are limited in their distribution by length of day and temperature; they flower only under short day conditions of 12 hours or less of sunlight. They are, thus, found in great abundance between latitudes 45° and 30°. One noteworthy exception is that they do not seem to naturalise generally in Japan (Wagner 1959). Ambrosia species occur only rarely in the British Isles (where the the name ragwort is used for species of Senecio L.). Several species are found in other European countries, including Germany, Belgium, France, Switzerland, Holland, Austria, Sweden, and Italy. Ragweeds also occur, and are a dermatological hazard, in Argentina (Bozzola & Valentinetti 1955), and in the [former] USSR (Samo Ilov & Terekhova-Uvarova 1974). Two introductions of local interest are along the railroad tracks of British Columbia, Canada, and on the World War II invasion beaches of Normandy.

Allergic contact dermatitis from (mainly the pollen of) Ambrosia species is the subject of an extensive literature dating at least from Hannah (1919) and Sutton (1919), and reviewed by Mitchell (1969). A distinction was made by Brown et al. (1931) between contact dermatitis from the oleoresin of the pollen and the Type I hypersensitivity from the proteinaceous fraction of the pollen. Sweitzer & Rusten (1938b) have reported the occurrence of positive patch and scratch tests to ragweed pollen in the same patient.

The principal contact sensitisers are sesquiterpene lactones (Mitchell et al. 1971a); the exact nature of the atopens has yet to be clarified. Cross-sensitivity between Ambrosia species and other members of the family Compositae was reported by Brunsting & Williams (1936), and elaborated upon by Shelmire (1939a) and Mackoff & Dahl (1951). The biochemical basis of the cross-sensitivity is the presence of structurally related sesquiterpene lactones in the plants (Mitchell & Dupuis 1971). Persistence of ragweed dermatitis may result from contact with fomites: Wyse & Mallock (1970) note that ragweed pollen may be brought into the house with a Christmas tree; grain and hay may be contaminated with ragweed oil or other sesquiterpene lactone containing plants. Jordan et al. (1942) report ragweed dermatitis among workers in the flour and grain industries.

Frain-Bell & Johnson (1979) observed positive patch test reactions to the oleoresin from an unspecified Franseria species in 9 from 55 patients with the photosensitivity dermatitis and actinic reticuloid syndrome.

Becker & O'Brien (1959) and Tan & Mitchell (1968) observed contact sensitivity to this plant. Mitchell et al. (1971a) investigated contact sensitivity to the false ragweed and the short ragweed (see A. artemisiifolia L. below). In an investigation of "weed dermatitis", this species produced positive patch test reactions in 15 of 25 patients tested. Some of the patients were also sensitive to other Ambrosia as well as Helenium L., Iva L., Parthenium L., and Xanthium L. species (Mackoff & Dahl 1951). Cross-sensitivity has also been observed between A. acanthicarpa and Artemisia L., Chrysanthemum L., and Tanacetum L. species (Mitchell 1972).

A number of sesquiterpene lactones, including artemisiifolin, chamissonin, confertiflorin, desacetylconfertiflorin, and psilostachyin C have been isolated from this franserioid species growing in various locations in the USA (Yoshioka et al. 1973). The species ranges west and south from the Dakotas and Nebraska to Washington, Oregon, California, and New Mexico.

Ambrosia ambrosioides Payne

(syn. Franseria ambrosioides Cav.)

This taxon is found in the south-west of USA and in Mexico. The sesquiterpene lactones damsin and franserin have been reported from the plant (Yoshioka et al. 1973).

Ambrosia arborescens Mill.

(syns Ambrosia fruticosa Medik., Xanthium fruticosum L.f.)

Altimisa

This South American taxon is found in Ecuador, Columbia, Peru, and Bolivia. The sesquiterpene lactones coronopilin, damsin, psilostachyin, and psilostachyin C have been reported from the plant (Yoshioka et al. 1973).

Ambrosia artemisiifolia L.

(syn. Ambrosia elatior L.)

Short Ragweed, Common Ragweed, Low Ragweed

This species is found almost throughout the United States, extending in distribution from the Atlantic to the Pacific coast, throughout southern Canada, and as far south as southern Florida, northern California, and Cuba. Short ragweed is a complex and variable species consisting of a number of genotypes which, under natural environmental conditions, tend to segregate out (Wodehouse 1971).

In various reports, 14 of 18 (Brunsting & Anderson 1934), 37 of 40 (Brunsting & Williams 1936), 23 of 50 (Shelmire 1939a), and 23 of 25 patients (Mackoff & Dahl 1951) who had "weed dermatitis" showed positive patch test reactions to this species. Krook (1977) observed positive patch test reactions to this species in 2 of 4 patients with occupational contact dermatitis to lettuce (Lactuca sativa L.). Burry & Kloot (1982) believe that Australian bush dermatitis is caused by airborne dusts from members of the Compositae, formed by the breakdown of senescent tissues in the hot, dry conditions of the bush, and that ragweed (A. artemisiifolia) is one of the specific elicitors of this condition. These authors also give distribution maps for Ambrosia species in Australia.

Mitchell et al. (1971a) investigated contact sensitivity to the short ragweed and the false ragweed. Patients were found to be sensitive to a number of sesquiterpene lactones (desacetylconfertiflorin, psilostachyin, psilostachyin B, psilostachyin C, ambrosiol, damsin, tamaulipin A, tamaulipin B, isabelin, & reynosin) as well as to burweed marshelder and feverfew (unidentified species if Iva L. and Chrysanthemum L. respectively) and liverworts belonging to the genus Frullania Raddi (fam. Jubulaceae).

Tan & Mitchell (1968) and Thune & Solberg (1980) observed positive patch test reactions to the oleoresin from this species and other members of the Compositae in two photosensitive and lichen allergic patients. Positive patch test reactions to the oleoresin of the species were observed by Frain-Bell & Johnson (1979) in 10 from 55 patients with the photosensitivity dermatitis and actinic reticuloid syndrome.

Several sesquiterpene lactones, including ambrosin, coronopilin, damsin, isabelin, and psilostachyin have been reported from this species (Yoshioka et al. 1973). Inayama et al. (1974) reported the isolation of ambrosic acid, a sesquiterpenoid compound without a γ-lactone ring, from the pollen and other parts of this species. They described this compound as "irritant" but did not state how this activity was discovered.

Ambrosia artemisioides Meyen & Walp.

Yoshioka et al. (1973) reported that no sesquiterpene lactones could be detected in this species which grows in southern Peru and northern Chile.

Ambrosia bidentata Michaux

Lance-leaved Ragweed

This species was described as a plant of high sensitising capacity in the production of "weed dermatitis" in the southern United States (Shelmire 1940). Yoshioka et al. (1973) record that no crystalline lactones could be isolated from this species.

Ambrosia bryantii Payne

(syns Acanthambrosia bryantii Rydb., Franseria bryantii Curran)

This species ranges from central to southern Baja California, Mexico. It is remarkable for the long thorny spines of the fruiting involucre (Payne 1964).

Ambrosia camphorata Payne

(syn. Franseria camphorata Greene)

This extremely variable franserioid species from Baja California and western Sonora has been found to contain isoalantolactone (Yoshioka et al. 1973), a sesquiterpene lactone with proven sensitising potential (Stampf et al. 1978).

Ambrosia canescens A.Gray

(syn. Franseria canescens Rydb.)

This franserioid species, which is found in east-central Mexico and into Arizona, has been found to contain the potentially allergenic sesquiterpene lactone canambrin (Yoshioka et al. 1973).

Ambrosia castanensis Payne

The potentially allergenic sesquiterpene lactone artemisiifolin has been reported from this species growing in Coahuila, Mexico (Yoshioka et al. 1973).

Ambrosia chamissonis Greene

(syn. Franseria chamissonis Less.)

This somewhat variable franserioid species is found along the Pacific coasts of North and South America. It has been found to contain a number of potentially allergenic sesquiterpene lactones including chamissonin and costunolide.

Ambrosia cheiranthifolia A.Gray

No sesquiterpene lactones could be detected in this species from southern Texas (Yoshioka et al. 1973).

Ambrosia chenopodiifolia Payne

(syn. Franseria chenopodiifolia Benth.)

This franserioid Baja Californian species has been found to contain the sesquiterpene lactones damsin, psilostachyin, and psilostachyin C (Yoshioka et al. 1973).

Ambrosia confertiflora DC.

(syns Franseria tenuifolia Harvey & A.Gray, Ambrosia fruticosa DC.)

This extremely variable species is found in south-western USA, Mexico, Puerto Rico, and on the Hawaiian Islands of Oahu and Molokai. It has been reported to yield a number of sesquiterpene lactones including confertiflorin, reynosin, and parthenolide (Yoshioka et al. 1973).

Ambrosia cordifolia Payne

(syn. Franseria cordifolia A.Gray)

This taxon is found from southern Arizona, USA into northern Mexico and Baja California. The sesquiterpene lactones cordilin, psilostachyin B, and psilostachyin C have been isolated from the plant (Fischer et al. 1979).

Ambrosia cumanensis Kunth

A number of sesquiterpene lactones, including ambrosin, coronopilin and damsin have been reported from this Central American species (Yoshioka et al. 1973).

Ambrosia deltoidea Payne

(syn. Franseria deltoidea Torr.)

This franserioid species, which grows in southern Arizona, USA, Sonora, and Baja California, has been reported to contain the sesquiterpene lactones damsin and psilostachyin C (Yoshioka et al. 1973).

Ambrosia dumosa Payne

(syn. Franseria dumosa A.Gray)

This franserioid taxon is found in arid regions of the southern United States, extending into Sonora and Baja California. A number of sesquiterpene lactones including chamissonin, coronopilin, and psilostachyin have been isolated from the plant (Yoshioka et al. 1973).

Ambrosia eriocentra Payne

(syn. Franseria eriocentra A.Gray)

No sesquiterpene lactones could be detected in this franserioid species which grows in southern Arizona and arid regions of Nevada, Utah, and California (Yoshioka et al. 1973).

Ambrosia grayi Shinn.

(syns Franseria tomentosa A.Gray, Franseria grayi Nelson)

No sesquiterpene lactones have been detected in this species (Yoshioka et al. 1973).

Ambrosia hispida Pursh

The sesquiterpene lactones ambrosin and damsin have been reported from this species which is found in the Florida Keys and Caribbean region (Yoshioka et al. 1973). Herz et al. (1981c) noted that flowering A. hispida is an excellent source of ambrosin and damsin, but that the vegetative plant yields principally damsinic acid, neoambrosin, and an ester of damsinic acid with 2-hydroxyambrosin.

Ambrosia jamaicensis Payne (unpublished)

Potentially allergenic sesquiterpene lactones have been reported from a plant given this name.

Ambrosia linearis Payne

(syns Gaertneria linearis Rydb., Franseria linearis Rydb.)

No sesquiterpene lactones could be detected in this franserioid species (Yoshioka et al. 1973).

Ambrosia maritima L.

This taxon is included in Ambrosia artemisiifolia L. by some authorities. It has been found to contain the potentially allergenic sesquiterpene lactones ambrosin and damsin (Yoshioka et al. 1973).

Ambrosia peruviana Willd.

This species from central South America has been reported to contain the sesquiterpene lactones tetrahydroambrosin, peruvinin, peruvin, and psilostachyin C (Yoshioka et al. 1973).

Ambrosia polystachya DC.

Potentially allergenic sesquiterpene lactones have been reported from this species.

This species is found throughout North America from southern Canada to central Mexico. It occurs in several slightly different forms which are regarded by some taxonomists as different species (Wodehouse 1971). Different populations of the plant yield different sesquiterpene lactones (Potter & Mabry 1972, Yoshioka et al. 1973) many of which are proven or potential contact allergens (Mitchell & Dupuis 1971).

"Weed dermatitis" from this plant was reported by Brunsting & Williams (1936), Shelmire (1939a, 1940), and by Burry et al. (1973) in Australia. Also in Australia, Burry (1979), Turner (1980), and Burry (1980b) observed positive patch test reactions to this species in patients with "fleabane dermatitis" (caused by Conyza bonariensis Cronq.), in a patient with dermatitis attributable to picking firebush (Ixodia achillaeoides R.Br.) and wild artichoke (Cynara cardunculus L.), and in a patient with dermatitis caused by Gaillardia aristata Pursh respectively. Burry (1980a) also observed a weakly positive patch test reaction to this species in a 75 year old female who had been admitted to hospital because of a severe exacerbation of a condition that had been diagnosed four years previously as photodermatitis.

The presence of Δ-3-carene, and α- & β-pinenes in this species (Potter & Mabry 1972) may explain the cross-sensitivity between turpentine and ragweed dermatitis described by Fisher (1952).

Ambrosia pumila A.Gray

(syn. Franseria pumila Nutt.)

This species from San Diego County, California and northern Baja California has been found to contain desacetylconfertiflorin, psilostachyin, and psilostachyin C (Yoshioka et al. 1973).

Ambrosia tenuifolia Spreng.

This Argentinian species has been reported to contain the sesquiterpene lactones confertin and psilostachyin (Yoshioka et al. 1973).

Ambrosia tomentosa Nutt.

(syn. Franseria discolor Nutt.)

No sesquiterpene lactones could be detected in this species (Yoshioka et al. 1973).

Ambrosia trifida L.

(syn. Ambrosia aptera DC.)

Giant Ragweed, High Ragweed, Tall Ragweed

This species is of low sensitising capacity (Brunsting & Williams 1936, Shelmire 1939a) and rarely produces positive patch test reactions in patients sensitive to members of the Compositae. Frain-Bell & Johnson (1979) observed positive patch test reactions to the oleoresin of this species in 4 from 54 patients with the photosensitivity dermatitis and actinic reticuloid syndrome.

Yoshioka et al. (1973) observed that no sesquiterpene lactones have been reported from this species.